Apparatus and process for coating by vapor deposition



Jan. 5, 1954 p, GQDLEY, 2 2,665,225

APPARATUS AND PROCESS FOR COATING BY VAPOR DEPOSITION Filed. ApTi'l 27,1950 FIG.

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ATTORNEY Patented Jan. 5, 1954 APPARATUS AND PROCESS FOR COATING BYVAPOR DEPOSITION Philip Godley 2nd, Lexington, Mass, assignor toNational Research Corporation, Cambridge, Mass, a corporation ofMassachusetts Application April 27, 1950, Serial No. 158,493

5 Claims. 1

This invention relates to coating and more particularly tovacuum-deposition coating of the type wherein a metal, such as aluminum,is vaporized in a vacuum and the vapors thereof are condensed on asubstrate which is moved past the source of the metal vapors.

This application is in part a continuation of the copending applicationof Chadsey et a1., Serial No. 117,124, filed September 22, 1949.

In vacuum-deposition coating it is desired to have a relatively largesource of the metal, such as aluminum, so that large areas of thesubstrate may be coated Without shutting down the operation of thecoating device. It is also desirable to have a relatively smallevaporating area heated to a high temperature so as to decrease theamount of radiant heat which is transferred to the substrate per gram ofaluminum coated on the substrate.

Accordingly, it is a principal object of the present invention toprovide an improved process for the vapor-deposition coating ofsubstrates with metals, such as aluminum, wherein a large supply of themetal is maintained in the vacuum chamber, and a relatively smallpercentage of the metal is heated to a high temperature to obtain arapid evaporation thereof.

Another object of the present invention is to provide a coating processof the above type wherein a large supply of the. metal is maintained ata, relatively low temperature and a molten portion of the metal in thesupply is caused to travel from the low-temperature supply to ahigh-temperature evaporation zone where it is rapidly evaporated.

Another object of the invention is to provide such a process which canbe practiced with very simple forms of coating apparatus.

Still another object of the invention is to proparatus possessing theconstruction, combination of elements and arrangement of parts, and theprocess involving the several steps. the relation and theorder of one ormore of such steps with respect-to each of the others which areexempllfied in the following detailed disclosure. and

the scope of the application of which will be indicated in the claims.

For a, fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings wherein:

Fig. 1 is a diagrammatic, schematic, partially sectional view of onepreferred apparatus embodying the present invention;

Fig. 2 is a schematic, diagrammatic, isometric view of a portion of theapparatus of Fig. 1; and

Fig. 3 is a diagrammatic, schematic, isometric view of a modification ofa portion of the apparatus shown in Fig. 1.

In the practice of the present invention, there is provided a usualvacuum-coating chamber which can be evacuated to pressures in the micronrange. Associated with this chamber there is provided a low-temperaturezone for confining the metal to be coated, this low-temperature zonebeing preferably arranged to maintain the metal in a molten pool at atemperature somewhat above its melting point.

In a preferred form of the process of this invention, the metalcomprises aluminum, and for simplicity of description aluminum will bereferred to hereinafter. The large body of aluminum confined in thelow-temperature zone is preferably held in a container which issubstantially inert to molten aluminum at relatively low temperatures onthe order of 800 C. There is also provided a high-temperatureevaporation zone where the aluminum may be heated to a high temperature,preferably above 1300 C. At this temperature the heat transmitted byradiation from a unit area of the evaporating aluminum surface to a unitarea of the substrate is less than the heat transmitted to a unit areaof the substrate by the vapors condensing thereon. For feeding themolten aluminum from the low-temperature zone to the evaporation zone,there is included a wick element which extends into the pool of moltenaluminum and is wet by the molten aluminum. The aluminum, which wets thewick element, travels from the low-temperature zone to thehigh-temperature zone where it is rapidly evaporated. This wick elementpreferably comprises a material which is relatively inert to aluminumat-high temperatures, and preferably comprises a nitride or a carbide.Of the carbides and nitrides, those of the group Iva and group Va metalcarbides and nitrides are preferred. The wick element also preferablyacts as a sup.- port. for the aluminum inthe high temperatureevaporation zone. 1

Referring now to Figs. 1 and 2, there is shown one preferred embodimentof the invention wherein there is provided a vacuum-tight housing whichdefines therewithin a vacuum chamber I2. Vacuum chamber I2 is arrangedto be evacuated to very low pressures, on the order of one micron, bymeans of a vacuum pumping system, schematically indicated at M. Withinthe vacuum chamber there is provided a means for supporting thesubstrate to be coated, this means comprising a first spool l6 and asecond spool l8 carrying therebetween the substrate 20. For holding therelatively large supply of metal, there is included a low-temperaturezone indicated generally at 22, while for evaporating the metal there isprovided a high-temperature zone generally indicated at 23. Thelow-temperature zone comprises a container 25 which may be formed ofcarbon or the like, this container confining a large body oflow-temperature molten aluminum 24. The aluminum 24 is arranged to bemain.- tained slightly above its melting point by means of an inductionheating coil 26. For feeding molten aluminum from the low-temperaturezone 22 to the high-temperature evaporation zone 23, there is provided awick element 28 which communicates between these two zones, aluminumtraveling along the wick element being indicated at 24a. The upper endof wick element 28, which constitutes the high-temperature evaporationzone 23, is arranged to be heated by an induction coil 30 whichpreferably maintains the aluminum 24a on the surface of the wick elementat a high temperature, on the order of 1300 C. or above.

For eliminating radiation from all but the tip of the wick 28, there isincluded a refractory shield 32 provided with a hole 34 through whichthe wick 28 extends into the evaporation zone. Power source 36 suppliescurrent to the two induction coils 26 and 30.

As can be seen from Fig. 2, the wick element 28 is preferably in theform of a long plate which has a length, extending transversely of thesubstrate, which is approximately equal to the width of the substrate.This particular arrangement has the advantage that the upper edge of thewick 28 constitutes a line source for evaporating aluminum at a hightemperature. Since this upper edge is substantially completely coveredby aluminum, which wets the wick element, there is substantially noradiation of heat to the substrate other than that radiating from theevaporating surface of the aluminum24a. Refractory shield 32 preventsradiation from the surface of the large low-temperature pool of aluminum24, and also prevents radiation from that aluminum 24a which is climbingup the wick 28 and which is not heated to the high evaporationtemperature.

In a preferred embodiment of the invention described above, thecontainer 25 preferably comprises carbon, or a mixture of carbon andgraphite, while the wick element 28 preferably com-' prises a carbonplate whose surface has been treated to form a carbide of a group N11 orgroup Va metal. One preferred method of forming such a carbide surfaceis to dip the plate under vacuum, into a molten bath of aluminum andzirconium, this molten bath forming a zirconium carbide surface on allportions of the carbon plate dipped into the bath. The current in theinduction coil 30 is preferably of a relatively low' frequency, so thatthe skin depth of the induced current is greater than the thickness ofthat portion of the carbon container which is between the induction coiland the wick element 28. With this arrangement, high induced currentsare produced in the wick element 28 adjacent the top thereof to heatthis top portion to a high temperature, on the order of 1300 C. orabove.

In lieu of precoating the wick element 28, zirconium metal may be addedto the aluminum 24 in the container 22, so that a zirconium carbide skinis formed on the surface of the wick element when in use. However, thissystem is less preferred than that wherein the wick element is firsttreated so as to form thereon the carbide surface. Other materials whichare Wet by aluminum may be utilized in place of a carbide of the groupIVa and group Va metals but, for reasons of expense, they are lesspreferred. Additionally, it has been found that the group IVa and groupVa metals are eminently satisfactory for this purpose and, of thesemetals, zirconium, because of its effectiveness and relative abundance,is preferred.

In the use of the Fig. 1 and Fig. 2 embodiments of the invention, thesubstrate 20 is mounted in the coating chamber [2, the chamber isevacuated to about one micron, and the aluminum 24 in the container 25is heated by the coil 26 to slightly above its melting point. The toppart of the wick 28 is then heated to a high temperature by means of theinduction coil 30. During this heating, the aluminum 24 commences toclimb, as indicated at 24a, up the surface of the wick element 28 andcompletely covers the upper end thereof. Since the upper end of thiswick is at a high temperature, the climbing aluminum 24a is heated to anequally high temperature and evaporates rapidly, the vapors traveling tothe substrate 20 where they are condensed to form a continuous shinyfilm of aluminum. In order to maintain a uniform thin coat, thesubstrate 20 is preferably moved at a speed commensurate with the amountof vapors created at the top of wick element 28. This speed ispreferably adjusted to give a coating of approximately .7 gram to 30square feet of substrate area. If desired, automatic controls of thetype described in the copending application of Philip Godley 2nd, SerialNo. 10,117, filed February 21, 1948, may be employed for controllingthe. coating thickness. 1

In an alternative embodiment of the invention, the high temperatureevaporation zone comprises a plurality of separate zones rather than asingle line zone as shown in Fig. 2. In Fig. 3 a plurality of wicksareprovided, these Wicks being preferably spaced transversely of the sheet,each wick coating a predetermined, longitudinal segment of the substrateas it is moved through the coating chamber. In Fig. 3, where likenumbers refer to" like elements of the Fig. 1 and Fig. 2 embodi-' ments,there is provided a plurality of wick ele ments 28a, each wick element28a comprising'a' generally cylindrical rod having a rounded topportion. In this embodiment of the invention, there is provided aplurality of holes 34a through which the upper ends of the wick element28c extend. The Fig. 3 embodiment of the invention spaced transverselyof the sheet so as to give a uniform coating. The materials utilized inthe Fig. 3 embodiment of the invention are prefer ably similar to thosediscussed in connection with" the Fig. 1 and Fig. 2 form ofthe'in'ventionl The expression group IVa and group Va metals is intendedto include those metals in groups IVa and Va on the Periodic Chart ofthe Atoms, Henry D. Hubbard, 1947 Edition, W. M. Welch ManufacturingCompany.

Since certain chan '25 may be made in the above process and apparatuswithout departing from the scope or" the invention herein involved, itis intended that all matter contained in the above description, or shownin the accompanying drawings, shall be interpreted as illustrative andnot in a limiting sense.

What is claimed is:

1. A process for coating a substrate with aluminum by vacuum-evaporatingsaid aluminum and condensing said aluminum on said substrate, saidprocess comprising the steps of providing a supply or" said aluminum ina vacuum chamber, evacuating said chamber, melting at least a portion ofsaid aluminum said supply of aluminum, providing a wick element havingat least a surface stratum comprising a compound selected from the classconsisting of the carbides and nitrides of titanium, zirconium, hafnium,vanadium, coluinbium and tantalum, confining said molten aluminum in alow-temperature pool in contact with said Wick element, heating saidWick element to a temperature higher than the temperature in said pool,evaporating by said high temperature that aluminum which wets said wick.l

element, shielding said substrat from radiation emanating from thelow-temperature pool of molten aluminum, and moving said substrate pastsaid wick element to coat aluminum on said substrate.

2. Apparatus for coating a substrate with aluminum by vacuum-evaporationof said aluminum and condensation of said aluminum on said substrate,said apparatus comprising means defining a vacuum-tight chamber, acontainer for confining said aluminum within said chamber, means formelting at least a portion of said confined aluminum, means comprising aplurality of Wick elements, said wick elements comprising at least asurface stratum selected from the group consisting of the carbides andnitrides of titanium, zirconium, hafnium, vanadium, columbium andtantalum, means for heating said Wick elements to a temperature higherthan the temperature of said molten aluminum, said container forconfining said aluminum being arranged to maintain at least a portion ofsaid molten aluminum in at least one pool in contact with said wickelements so that said wick elements are wet by said aluminum and saidaluminum is evaporated from said Wick elements by said high temperature,means comprising a radiation shield over said container, said wickelements extending from within said container to the exterior thereofand beyond said radiation shield, and means for moving said substratepast said Wick elements to coat aluminum on said substrate.

3. In an apparatus for coating a substrate with aluminum byvacuum-evaporation of the aluminum and condensation of the aluminumvapors on a substrate, an improved source of aluminum vapors comprisinga carbon container for holding molten aluminum, means comprising aradiation shield over said container, means for heating said aluminum tomaintain said aluminum in molten condition, a wick element which iswettable by molten aluminum, said wick element comprising at least asurface stratum selected from the group consisting of the carbides andnitrides of titanium, zirconium, hafnium, vanadium, columbium andtantalum, said wick element extending from within said container to theexterior thereof and beyond said shield, and means for heating said wickelement to a temperature higher than the temperature of said moltenaluminum.

4. Apparatus for coating a substrate with aluminum by vacuum-evaporationof said aluminum and condensation of said aluminum on said substrate,said apparatus comprising means defining a vacuum-tight chamber, acontainer for confining said aluminum within said chamber, means formelting at least a portion of said confined aluminum, means comprising awick element, said wick element comprising at least a surface stratumselected from the group consisting of the carbides and nitrides oftitanium, zirconium, hafnium, vanadium, columbium and tantalum, meansfor heating said wick element to a ternperature higher than thetemperature of said molten aluminum, said container for confining saidmolten aluminum being arranged to maintain at least a portion of saidmolten aluminum in a pool in contact with said wick element so that saidwick element is wet by said aluminum and said aluminum is evaporatedfrom said wick element by said high temperature, a radiation shield oversaid container, said wick element extending from within said containerto the exterior thereof and beyond said radiation shield, and means formoving said substrate past said wick element to coat aluminum on saidsubstrate.

5. The apparatus of claim 4 wherein said wick element comprises aplatelike member, one long edge of which is positioned in said pool ofaluminum and the other long edge of which extends above the shield so asto be in position to radiate aluminum vapors to said substrate.

PHILIP GODLEY 2ND.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,153,786 Alexander et a1 Apr. 11, 1939 2,159,297 Shover May23, 1939 2,351,536 Osterberg et a1 June 13, 1944 2,363,781 Ferguson Nov.28, 194:4 2,382,432 McManus et al. Aug. 14, 1945 2 384,578 Turner Sept.11, 19%5 2,387,970 Alexander Oct. 30, 1945 2,450,853 Colbert et a1. Oct.5, 1948

1. A PROCESS FOR COATING A SUBSTRATE WITH ALUMINUM BY VACUUM-EVAPORATINGSAID ALUMINUM AND CONDENSING SAID ALUMINUM ON SAID SUBSTRATE, SAIDPROCESS COMPRISING THE STEPS SO PROVIDING A SUPPLY OF SAID ALUMINUM IN AVACUUM CHAMBER, EVACUATING SAID CHAMBER, MELTING AT LEAST A PORTION OFSAID ALUMINUM IN SAID SUPPLY OF ALUMINUM, PROVIDING A WICK ELEMENTHAVING AT LEAST A SURFACE STRATUM COMPRISING A COMPOUND SELECTED FROMTHE CLASS CONSISTING OF THE CARBIDES AND NITRIDES OF TITANIUM,ZIRCONIUM, HAFNIUM, VANADIUM, COLUMBIUM AND TANTALUM, CONFINING SAIDMOLTEN ALUMINUM IN A LOW-TEMPERATURE POOL IN CONTACT WITH SAID WICKELEMENT, HEATING SAID WICK ELEMENT TO A TEMPERATURE HIGHER THAN THETEMPERATURE IN SAID POOL, EVAPORATING BY SAID HIGH TEMPERATURE THATALUMINUM WHICH WETS SAID WICK ELEMENT, SHIELDIGN SAID SUBSTRATE FROMRADIATION EMANATING FROM THE LOW-TEMPERATURE POOL OF MOLTEN ALUMINUM,AND MOVING SAID SUBSTRATE PAST SAID WICK ELEMENT TO COAT ALUMINUM ONSAID SUBSTRATE.